1. Field of the Invention
The invention involves an aqueous developer and a process for preparing flexographic printing forms by imagewise exposure of a recording material, washing off the uncrosslinked portions of the layer with the special aqueous developer, and drying the resulting treated recording material.
2. Description of Related Art
The use of photopolymerizable plates for preparing flexographic printing forms is known. The printing surface is produced by imagewise exposure of a layer that is photopolymerizable by actinic radiation and by subsequently removing the unexposed, unphotopolymerized areas of the printing plate. Aqueous solutions or organic solvents are usually employed as the developer, depending on the binder.
More and more aqueous developers have been used in recent years due to their lower environmental impact and decreased health hazard. Examples of aqueous developers being used are aqueous solutions of alkali metal hydroxides or carbonates and aqueous solutions of ionic or nonionic surfactants. Developers containing surfactants are described in JP 6-297829.
The washoff process is conducted mostly in washoff devices with rotating brushes of various materials. Inorganic developers as well as developers containing surfactants often leave deposits of the washed-off material on the brushes, the brush boards, and the walls of the device. Such deposits, which are often tacky polymer filaments, are located particularly at the base of the brush. The quality of the washoff process is impaired by these deposits. As this is not acceptable, the developer must be changed prematurely and the washoff device cleaned, which is often costly in time and labor. In addition, the deposits on the printing form lead to poor, smudged print images, particularly in the negative lines and between the screen elements.
Therefore, the problem involved in the present invention was to improve the washoff process for aqueous-developable flexographic printing plates, in particular, to prevent deposits of photopolymer material in the processor, in the brushes, and on the printing form surface.
This problem is solved by an aqueous developer for photopolymerizable flexographic printing plates containing at least one trialkali salt of an N-alkylarnidocarboxymethylene-Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-tris(carboxymethylene)ethylenediamine having a long-chain aliphatic hydrocarbon radical in the N-alkylamido group, and by a process for preparing flexographic printing forms with the use of this developer.
Surprisingly, the developer of the present invention forms homogeneous dispersions of the washed-off photopolymer materials and causes only slight deposits in the processor, on the processor brushes, and on the processor walls, at, simultaneously, a higher washoff rate. Also, at a higher developer load, there are no tacky deposits that are difficult to remove; instead, fine, mostly brittle filaments are formed and are easy to remove. The surfaces of printing forms prepared with the developer of the present invention are free of deposits and thus, produce satisfactory printing results.
These advantages of the developer of the present invention were all the more surprising, because similar surfactants, such as, for example, coconut acid diethanolamide disclosed in JP 6-297829 and lauroyl ethylenediamine triacetate, described in Inform, Vol. 6, No. 10, (October 1995), cause very heavy contamination in the brushes and washoff device.
The present aqueous developer contains, as a component essential to the invention, at least one trialkali salt of an N-alkylamidocarboxymethylene-Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-tris(carboxymethylene)-ethylenediamine having a long-chain aliphatic hydrocarbon radical in the N-alkylamido group. These ethylenediamine triacetate alkylacetamides are used preferably in quantities of 0.1 percent by weight or more, 0.2 to 5 percent by weight being particularly preferred. In particular, 0.2 to 3 percent by weight are advantageous. Sodium salts are the preferred alkali salts. Preferred long-chain aliphatic hydrocarbon radicals in the N-alkylamido group are those having 6 to 22 carbon atoms in the chain, such as, for example, in the hydrocarbon chains (including the carboxyl group) of caprilic, capric, lauric, myristic, palrnitic, and oleic acid. Mixtures of the preceding are also suitable. Mixtures based on coconut fatty acids are especially preferred. Ethylenediamine triacetate-coconut alkylacetamide trisodium salt is particularly preferred.
The developer of the present invention can contain an amino alcohol and/or an amino carboxylic acid as an additive, because, these increase the washoff rate. Triethanolamine, tris(hydroxy-methyl)aminomethane, and nitrilotriacetic acid are preferred. The quantity of these additives is preferably 0.05 to 1.0 percent by weight, in particular 0.1 to 0.6 percent by weight.
Furthermore, fatty acids can be added to the developer, preferably 0.05 to 1.0 percent by weight, in particular 0.1 to 0.5 percent by weight. Oleic acid is particularly preferred. The addition of fatty acids is advantageous, as this makes the polymer particles even easier to remove.
The developer of the present invention can contain a polyamino acid as another additive. Polyaspartic acid is preferred, The added quantity is preferably 0.05 to 2.0 percent by weight, particularly 0.1 to 1.0 percent by weight.
The present developer is prepared by dissolving the appropriate quantities of the components in water before the development process. However, it is also possible to prepare a concentrate of the components and to dilute this with water to the required concentration just before the start of the washoff process. Such concentrates usually contain water, 10 to 90 percent by weight, preferably 30 to 60 percent by weight, of at least one ethylenediarnine derivative essential to the invention, 5 to 50 percent by weight, preferably 15 to 35 percent by weight, of at least one amino alcohol, and 5 to 50 percent by weight, preferably 15 to 35 percent by weight, of at least one fatty acid. The concentrate is diluted with water to produce a useful developer solution. This solution contains preferably 0.1 to 10 percent by weight, particularly 0.5 to 5 percent by weight of the concentrate. Particularly preferred are 0.5 to 3 percent by weight of the concentrate.
The developer of the present invention can be used in all conventional washoff devices, preferably in high-quality steel devices. All current brush materials, such as, for example, nylon, polyester, or polypropylene, can be used. The washoff process is conducted preferably at an elevated temperature, usually at xe2x89xa730xc2x0 C., in particular at 5xc2x0 C.
The pH value of the developer is usually 8-11, preferably 8-10.
The present developer is suitable for developing aqueous-developable recording materials having photopolymerizable layers. These layers contain preferably hydrophilic polymers, optional hydrophobic polymers, photopolymerizable monomers, photoinitiators, and added auxiliaries, such as plasticizers, fillers, stabilizers, dyes, inhibitors, etc.
Aqueous-developable printing plates contain, as an essential binder, polymers having hydrophilic groups, such as, for example, hydroxyl, carboxyl, amino, nitrile, sulfate, and phosphate groups, or combinations of these groups. Examples are polyvinyl alcohols, polyvinyl acetates, copolymers of (meth)acrylic acid and (meth)acrylates, cellulose derivatives, polyesters, polyamides, carboxylated butadiene/acrylonitrile polymers, core-shell polymers having hydrophilic groups in the shell polymers, or block copolymers having hydrophobic and hydrophilic groups. Also useful are mixtures of these various polymers or mixtures with hydrophobic binders, such as thermoplastic elastomer copolymers, particularly with thermoplastic. elastomeric block copolymers. Linear and radial polystyrene/polybutadiene/polystyrene or polystyrene/polyisoprene/polystyrene block copolymers are preferred. Aqueous-developable flexographic printing plates are described in U.S. Pat. No. 5,175,076, European Publication EP-A 0 784 232, U.S. Pat. Nos. 4,621,044, 4,177,074, and 5,075,192.
The photopolymerizable materials can be applied onto commercial supports and exposed imagewise by current methods. After being developed with the developer of the present invention, the printing forms are dried, followed optionally by a chemical posttreatment and/or postexposure.